1. Field of the Invention
The present invention relates to a backlight unit of a liquid crystal display device, and more particularly, to a backlight unit of a liquid crystal display device using a cold cathode fluorescent lamp that is capable of realizing pure colors and that has a broad color reproduction range.
2. Discussion of the Related Art
A liquid crystal display device is a flat display device of a wide ranging variety of uses from that of office equipment to computer monitors to large-sized televisions due to ever-developing process and drive technologies. The liquid crystal display device controls light transmittance of a liquid crystal material through application of an electric field, thereby displaying a picture. A liquid crystal display device includes a liquid crystal display panel in which liquid crystal cells are arranged in a matrix pattern and a drive circuit for driving the liquid crystal display panel.
A liquid crystal display panel may be formed by bonding a thin film transistor substrate, on which thin film transistor arrays are formed, and a color filter substrate, on which color filter arrays are formed, with a liquid crystal layer therebetween.
M number of data lines and n number of gate lines substantially perpendicularly cross each other in the thin film transistor array substrate of the liquid crystal display panel, and accordingly, m×n number of liquid crystal cells are arranged in a matrix pattern. A thin film transistor is connected at each crossing of the data lines and the gate lines, and supplies data voltages applied through the data line to a pixel electrode of the liquid crystal cell in response to scan pulses of the gate line.
A black matrix, color filters, and a common electrode may be formed on a color filter substrate. The liquid crystal cell rotates liquid crystal having dielectric anisotropy by application of a potential difference between the data voltage supplied to the pixel electrode and the common voltage supplied to the common electrode. Thus, light transmittance is controlled.
Polarizers having optical axes substantially perpendicularly crossing each other may be adhered to the thin film transistor substrate and the color filter substrate of the liquid crystal display panel. An alignment film which determines a pre-tilt angle of the liquid crystal may further be formed on the inner surface thereof which is in contact with the liquid crystal layer. A storage capacitor may be further formed in each liquid crystal cell. The storage capacitor may be formed between the pixel electrode and the pre-stage gate line or between the pixel electrode and the common line, thereby uniformly maintaining the data voltage charged in the liquid crystal cell.
The drive circuit of the liquid crystal display device includes a data drive circuit and a gate drive circuit. The data drive circuit supplies data voltages to the data lines and the gate drive circuit sequentially supplies scan pulses to the gate lines to select the horizontal line of the liquid crystal cell to which the data voltage is supplied.
The liquid crystal display device is not a self luminous display device, thus a separate light source such as a backlight is required.
The lamps used as the backlight may include a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and a light emitting diode (LED) by way of non-limiting example.
FIG. 1 is a diagram representing a cross section of a related art CCFL among the foregoing lamps.
Referring to FIG. 1, the CCFL includes a glass tube 2; electrodes 4 located at both ends of the inside of the glass tube 2; and lead wires 6 which supply voltages from the outside to the electrodes 4. Further, phosphor 8 is spread over the internal surface of the glass tube 2, and a gas mixture of mercury, argon, neon and the like is injected into an interior space 10 of the glass tube 2.
If a voltage is supplied to the electrode 4 through the lead wire 6, electrons within the interior space 10 move at a high speed by being drawn to the electrode 4, thereby colliding with the electrode 4 so as to emit secondary electrons. The emitted secondary electrons collide with mercury atoms in the interior space 10 to irradiate ultraviolet rays, and the ultraviolet rays are converted into visible rays by the phosphor 8, thereby emitting light through the glass tube 2.
Phosphor 8 may include red R phosphor, green G phosphor and blue B phosphor, for example. Emitted color can be selected by controlling the mixing ratio of the R phosphor, G phosphor and B phosphor.
The liquid crystal display device might realize a desired color by irradiating the white light generated as a result of a combination of the R phosphor, G phosphor and B phosphor, to the color filter in accordance with the transmittance control of the liquid crystal.
FIG. 2 is a graph representing a phosphor wavelength spread over a CCFL of the related art, and a color filter wavelength of a color filter substrate of a liquid crystal display panel. Herein, the solid line represents a wavelength of each of R, G and B color filters, and the dotted line represents a phosphor wavelength. Further, the horizontal axis represents wavelength (nm), and the longitudinal axis represents percent of transmittance in accordance with each wavelength. Transmittance value is a percent of light transmittance based upon the assumption that maximum transmittance of the phosphor is 100%.
Referring to FIG. 2, the phosphor includes an R phosphor having a dominant wavelength of about 612 nm, a G phosphor having a dominant wavelength of about 545 nm, and a B phosphor having a dominant wavelength of about 450 nm. The lights generated in the R phosphor, G phosphor and B phosphor are mixed into a white light with transmittance controlled by the liquid crystal, as described above. Lights are radiated as a red light, green light and blue light by the R, G and B color filters, respectively.
The CCFL using the R phosphor, G phosphor and B phosphor, of which each has one dominant wavelength, is called a three-wavelength CCFL.
FIG. 3 is a related art chromaticity diagram illustrating a color reproduction range of the three-wavelength CCFL.
The three-wavelength CCFL of the related art has a color reproduction range of about 72% compared with an NTSC system (the color reproduction range in accordance with the NTSC system being the color television system established by US National Television System Committee (NTSC)) of about 100%. The CCFL can embody a wide variety of colors because of the phosphors' broad color reproduction range. However, a problem with the CCFL is that the various colors cannot be embodied because the three-wavelength CCFL does not satisfy the color reproduction range required by the NTSC system. Particularly, the three-wavelength CCFL of the related art has a wavelength of each phosphor located in the wavelength overlap area of the B color filter and the G color filter and in the wavelength overlap area of the G color filter and the R color filter, as shown in FIG. 2. Thus, it is difficult to realize pure blue and green colors. Also, the dominant wavelength of the R phosphor is about 612 nm, thus it is difficult to realize a dark red which is embodied in a wavelength substantially longer than 612 nm.